This invention relates to data and power communication between a surface location or station and a well tool in a well bore, and more particularly, to a system for transmitting power from a surface location to a downhole well tool or tools and for selectively communicating data signals between the surface location and a well tool for selective retrieval of data from a well tool.
As oil field technology advances, there is a need to have one or more parameter measurement tools on a more or less permanent or semi-permanent basis located in a well bore. The tools typically are desired to measure parameters such as pressure, temperature, fluid flow and density. Because such tools utilize electronics and have other components which may require attention from time to time, the tools can be made to be retrievably set in side pockets located along the well string. Alternatively, tools can be installed on a non-retrievable basis. Thus, the tools can be retrieved as deemed necessary from time to time.
A side pocket or side pockets or retrievable tools are respectively coupled by a single wire conductor cable to equipment at the surface location. In each side pocket is an inductor coupler component (usually a probe member) which receives an inductive coupler component located in a socket of the well tool. At present, the well tool obtains its electrical power from internal DC batteries. Data communication is effected via the conductor cable.
It is desirable to eliminate the requirement for non-retrievable electronics in the well tool and to communicate data from a well tool by supplying the power and communication data on a common conductor cable. Since the inductive coupling components basically define a transformer, the power and data communication functions must be accomplished with AC voltages and currents which are impressed upon a conductor cable line. The difficulty with this approach is that the power frequently should be as low as possible so that reactive currents in the conductor cable line are minimized. At the same time, data communication frequencies would like to be as high as possible to prevent undue restriction of data communication rates. These requirements present a conflict because choosing a power frequency too close to the communication frequencies will result in separation difficulty for low level current modulation communication frequencies in the presence of the high level power frequency. Secondly, separation of the address voltage frequencies (for different tools) from the power frequency becomes a significant problem downhole because of limited space and power available to implement sophisticated frequency filters.
The modulation and detection schemes used to implement the present invention addresses both of these issues and result in simple, highly reliable, simultaneous communications and power with the operating frequencies relatively close to one another.
In the present invention, at a surface location, a power source with a selected frequency is utilized to supply AC power via a conductor cable to one or more downhole inductor coupling component(s). A downhole well tool(s) with a cooperating inductor coupling component has a full wave rectifier which supplies operating DC power for the well tool. A well tool configured for use with the present invention includes a parameter sensing means for sensing pressure, temperature, fluid flow or density which senses the parameter and develops a digital signal as a function of the sensed parameter. The digital signal measurements are stored in an addressable gate array unit and are output when addressed in a current modulation transmission mode.
Also in the well tool is a level detector which senses the rectified output of the rectifier and detects an amplitude modulation of the rectified output signal for providing a digital address to the well tool gate array logic unit.
Hence, the downhole tool obtains DC power from a rectifier and detects an address signal modulated on the power frequency.
At a surface location, a discrete digital address for a given well tool is provided by a Frequency Shift Keying (FSK) communication signal which is impressed upon the power frequency. FSK frequencies of 600 and 1200 Hz are adequately high for data purposes and, of course, separate the 0 and 1 bits of a digital signal.
The amplitude modulation of the FSK communication signals on the power frequency is automatically demodulated by the power rectification in the well tool so that all that is required is to separate the modulation frequency from the power frequency ripple. This is accomplished by a filter capacitor selected so that the ripple frequency amplitude of the rectified signal is significantly smaller than th modulation frequency amplitude. A level detector then rejects the smaller amplitude ripple frequency and follows the larger amplitude modulation.
At the surface location the power frequency is selected as an exact multiple of the modulation frequencies (4.8 Khz) which minimizes the possibility of introducing a DC signal component into the line frequency.
Data transmission is effected by a well tool current modulation means which modulates the power current as a function of the data being transmitted by the well tool in response to the address signal. At the surface location, a current demodulation transforms the current signals into digital signals representative of the well tool signals.